Gas turbine engines generally include a gas generator which comprises a compressor for compressing air flowing rearward through the engine, a combustor in which fuel is mixed with the compressed air and ignited to form a high energy gas stream, and a turbine driven by the gas stream and connected to drive a rotor which in turn drives the compressor. Many engines further include a second turbine, known as a power turbine, located aft of the gas generator for extracting energy from the gas stream to drive a rotating load with variable pitch blades such as found in the propulsor of helicopters, ducted turbo-fan engines, and turbo-prop engines.
A recent improvement over the turbo-fan and turbo-prop engines described above is the unducted fan engine such as is disclosed in U.K. Patent Application No. 2,129,502 published May 16, 1984. In the unducted fan engine, the power turbine includes counterrotating rotors and turbine blades which drive counterrotating unducted fan blades located radially outward of the power turbine section of the engine.
The fan blades of the unducted fan engine are variable pitch blades to achieve optimum performance of the engine and thrust reversal. During operation, fuel efficiency of the engine can be optimized by varying the pitch of the blade to correspond to specific operating conditions.
In general, the environment of the engine has required that the actuating mechanism for controlling blade pitch utilize hydraulic actuators driving various types of gear arrangements to position the fan blades at desired locations or pitch angles. An exemplary form of pitch drive mechanism is illustrated in Wakeman et al. U.S. Pat. No. 4,657,484, issued Apr. 14, 1987 and assigned to the instant assignee, wherein the pitch of the fan blades is varied by a hydraulic actuator mounted inside the static power turbine support structure. The motion commanded from the actuator is first transmitted to the rotating member by a system of bearings and then to the blades by a system of gears and linkages mounted on the rotating member. It is desirable, in this type of system, to accurately position the two rows of counterrotating blades so that the pitch of each respective one of the fan blades not only matches the required thrust at various speeds but also produces accurate synchronization of speed of the two blade rows such that blade crossings occur at a precise position with respect to each other and the aircraft structure. In general, such systems utilize blade pitch to control thrust which in turn affects the rotational speed of the blades such that any slight variation in pitch control alters the precise position at which the blade crossings occur. A more detailed discussion of the mechanism and control system for controlling operation of the engine and for achieving desired blade pitch angles for the blades of each of the rows can be had by reference to Walker et al. U.S. Pat. No. 4,772,180, issued Sept. 20, 1988 and assigned to the assignee of the present invention.
Prior art systems which have relied on hydraulic controls for accurately positioning blade pitch sometimes function less than optimally in achieving the precision and bandwidth necessary to perform all of the control functions simultaneously. Furthermore, it has not been practical to provide back-up or alternate sources of hydraulic power to support the pitch control mechanism in case of failure of the primary system. Such failure thus requires shut-down of the pitch control system and default to a fixed emergency condition. Still further, it is believed that the hydraulic system efficiency is typically rather low, and it is desirable to increase efficiency of such system in order to improve specific fuel consumption, reduce peak demand and reduce the thermal load of the engine.
Some advances have been made in hydraulic controls by utilizing rotating hydraulic motors to drive gear trains which ultimately change and hold blade pitch position. The hydraulic motors are proportionately controlled by a small electrically operated pilot valve controlling the main control valve for the hydraulics. In other constructions, a variable swash plate may be used in the hydraulic motor to replace or supplement the power valve. The swash plate may be operated by an electric or hydraulic proportional actuator. In either of these embodiments, a single hydraulic pump operating from the engine powers both of the hydraulic motors. It is not believed practical to extend the hydraulic system beyond the engine envelope to achieve power source redundancy and a second full system or hydraulic pump requires an unacceptable weight penalty.
Although there have been attempts to implement electric drive systems for controlling the pitch of aircraft propellers during the World War II era, there appears to be no such implementation suitable for present-day aircraft. In general, prior attempts have used individual direct current motors switched on and off by relays running from the aircraft direct current power bus. Once the fan blade is driven to a desired location, the blade is held in that position by mechanical means since precise electrical servo controls were not available to maintain the fan blade position. Thus, the electrical drive system was used solely to change position of the fan blades, but not to actually maintain the position of the fan blades. Such prior art systems, however, all appear to be unacceptable in the type of aircraft engine to which this application is directed, due to the required reliability, operation of the control from unregulated power, precision and bandwidth of the control, and location in the oil sump region inside the engine.